I would go standard mixed until 20 (at least 18). Even with sword and board this will work nice.
At 18 you get spiritual attunement - important tool when balancing how many you are pulling and how your mana reflects the healers (try to end battles at about the same %). You'll also begin to see situations where Protection might actually help with higher teired skills... With a Priest healing (I must presume) and then a Lock Hunter Mage and Paladin you're looking at insane killrates and a sort of inability to keep any serious agro when fighting trash reguardless.
Pre-10 you tank, 10-18 you can still tank (either SnB or 2hander) or have the Voidwalker (extremely resilient tank at this level.) I will always prefer a real tank over a pet, but for say a boss fight in RFC it would be worthwhile to have the VW main tank.
So simple roadmap -> BoM 5/5, Imp Devotion Aura 5/5 gets you to 19.
Tacticset -> BoM You/Hunter/Pet, BoW Rest(level 14), use SotC for steady dmg output or SoR for threat, Devotion Aura. Single target specialization. SoR is agro.
This is where usually you'd respec full Ret, but if you're looking to tank for a set group of 5, Prot is the way to go. You got Retribution Aura at 16, RF at 16, and Conc at 20. All you "really" need. If you're all experienced you're going to notice agro being difficult to hold on trash, but just let it slide it's not entirely important still. Good idea to balance everyone's Manapools out is to pull multis (say VC or WC pull groups of 3-5) so the Lock can DoT up a few different targets, the Mage can play with AE, and the hunter (your worst threat producer right now) can toggle through targets with each ranged attack to lessen threat output.
Respec @ 20 -> Redoubt 5/5, Toughness 5/5, BoKings 1/1
Tacticset -> BoM Hunter, BoK You/H Pet, BoW Rest (*Succy gets BoM until 26, then BoW*), use SoR for damage or SoW (level 38) for mana, Retribution Aura. Group specialization. Consecrate, Retribution are agro. Have pet taunts off, use Rain of Fire and Blizzard on groups of more than 3.
You're going to want your Mage to be a rare spec which utilizes Improved Blizzard and a Frostbite/Shatter setup -- this will dramatically help with runners. Your Lock should be Affliction for the abilities to keep up with the rest of the group (for now, heh.) Your hunter should be Marks (for Aimed Shot, Scatter Shot, TSA). You'll get Salv at 26. It may come in handy to Salv the Mage and the Warlock (depending on what you're AEing, because casters tend to be harder to hold solid agro on since they're not getting the agro off of Ret aura.)
From 20 ->
Improved RF 3/3
Anticipation 5/5
Shield Specialization 1/3
Reckoning 5/5
Stoicism 2/2
Shield Specialization upped to 3/3
Blessing of Sanctuary 1/1
Holy Shield 1/1
Improved Holy Shield 2/2
Ardent Defender 2/5
Combat Expertise 5/5
Avenger's Shield 1/1
Don't know jack past that. When you finally get Improved Holy Shield 2/2, it's a good idea to start using BoSanct on yourself, finally, as the damage output in addition to the mitigation make it worthwhile in many cases.
While basing the group's makeup around the tank seems a bit unfair, the fact of the matter is Paladins excell at a certain type of tanking which is multis - they're really no excuse to do singles and have Demo Locks and Fire Mages... or Rogues at all unless you want to maybe PVP later... but do whatever.
Hope some of that helped at least. =)